The development of the OCaml-Rust interface is a bit organic. `ocaml-interop` aims to be a low-level but safe interface. At Inria we worked with the author to integrate ideas about using Rust's type system to safely work with a GC. We have more ideas to make it better; if someone wants to commit engineering time in this area they should feel encouraged to contact me.
the readme of ocaml-interop says it's "inspired by" ocaml-rs
while ocaml-rs says it "uses ocaml-interop behind the scenes" and "also exports an interop module, which is an alias for ocaml_interop and the two interfaces can be combined if desired"
from a cursory look over ocaml-rs seems possibly the one to use, as the more comprehensive project